At present, different types of implantable hearing prostheses are available for cases of total or partial hearing loss. Said devices are based on principles of electromagnetic, mechanical, piezoelectric or electric actuation.
The cochlear implant is a hearing prosthesis capable of restoring hearing in deaf people or people with hypoacusis, whose auditory nerve remains functional. More specifically a cochlear implant is based on a transducer which transforms the acoustic signals picked up by a microphone in electrical signals that stimulate the auditory nerve.
A fundamental element of the cochlear implant is the “sound processor”, which determines the electrical stimulation pattern which will be delivered through the electrodes of the implant. Said stimulation pattern varies according to the sound conditions of each situation. Furthermore, the electrodes are introduced in the scaly tympani of the “snail shell” or cochlea of the inner ear to finally transmit said stimuli to the auditory nerve, and from here to the brain.
The sound processors currently existing in the market aim to reproduce the information processing performed by a healthy ear. Unfortunately, said processors ignore a fundamental aspect of this processing that could be crucial for communication in noisy environments: the involuntary control over the operation of an ear exercised by the sounds received through the opposite ear (hereinafter, contra-lateral control). Perhaps for this reason, the users of cochlear implants show great difficulty for communicating in noisy environments, such as the roads and streets, premises with a lot of people such as cafeterias, bars and restaurants, etc.
In recent years, Blake S. Wilson (Duke University, USA) has led several research projects funded first by the National Institutes of Health and currently by the company MED-EL GmbH to design sound processors for cochlear implants which faithfully reproduce the operation of the healthy ear (i.e. bio-inspired systems) with the aim of improving the efficacy of the implant in noisy environments. Its solution consisted of designing a prototype of sound processor based on “computational models” of the response of the auditory nerve designed by Enrique A. Lopez-Poveda (University of Salamanca, Spain), who was incorporated in 2002 as a consultant to Blake S. Wilson's project. Reinhold Schatzer also participated in the team as engineer, currently at MED-EL Headquarters (Austria).
The operation of the bio-inspired processor and its results have been published in several documents (Wilson et al., “Two new directions in Speech Processor design for cochlear implants”, 2005; “Possibilities for a closer mimicking of normal auditory functions with cochlear implants”, 2006, “Use of auditory models in developing coding strategies for cochlear implants”, 2010) and in the doctoral thesis doctoral of Reinhold Schatzer (“Novel concepts for stimulation strategies in cochlear implants”, University of Innsbruck, 2010), but the research continues.
On the other hand, it is known that in “natural hearing”, the operation of each one of the ears depends on the operation of the opposite ear, so that the brain conjugates the information that reaches it from both ears (“binaural” hearing). At this point, we should mention the existence of what is known as “bilateral cochlear implant”. In this case, the user has two cochlear implants installed, one for each ear. A drawback of this type of implants is that the control and mode of operation of the implants is totally independent from one another. In other words, they lack the contra-lateral control present in healthy ears that provides the hearing to normal-hearing persons in noisy environments, hence their efficacy and performance in these noisy environments are clearly subject to improvement.
In this sense, the document US2012/0093329 uses a detection algorithm based on the inter-auricular intensity difference and processes sounds precisely on the detected inter-auricular intensity difference. This means that the sound processing technique requires from a prior detection on the intensity difference. A similar approach was disclosed by WO2010/022456A1 where the sound processing system described is also based on a detection algorithm; this detection algorithm uses a SNR signal-to-noise ratio in order to enhance the signal with respect to the noise detected. Another approach is that of US2009/0304188A1 where directional microphones are used to determine those signals considered to be relevant and perform a delay of said signals with respect of the rest of signals. All of the documents cited are based on, and therefore require, a priori detection or determination which requires more processing resources and transform signals in a way that the acoustic relevant signals may be deteriorated.
Other developments known in the art are not based on prior detection but on a time based alignment of the acoustic signals detected, in this sense the document published as US2004/0172101A1 describes a peak-derived timing stimulation strategy for a multi-channel cochlear implant, where the signals picked up by each of the two microphones are processed to align in time and
Some current hearing aids (e.g., Oticon®) feature bilateral wireless synchronization to match the hearing aids' noise reduction and directionality features. They also feature binaural coordination to coordinate the volume and program changes between the two devices (i.e., to make adjustments to the two devices by adjusting just one). Oticon® claims that their binaural hearing aids preserve inter-aural differences and help the user organize the sound scene, reducing listening effort. Due to proprietary right issues, the sound processing behind these claims is unknown to the present authors. Independent scientists, however, have confirmed some of the benefits claimed by the hearing-aid manufacturer (Wiggins I M, and Seeber B U, “Linking dynamic-range compression across the ears can improve speech intelligibility in spatially separated noise”, J. Acoust. Soc, Am. 133:1004-1016). Sound processing by hearing aids particularly frequency-dependent dynamic range compression, does not differ significantly from sound processing by a cochlear implant.
Therefore, the technical problem posed here is that current bilateral cochlear implants and the sound processing techniques developed do not provide an effective solution for the hearing and communication of deaf users in noisy environments, such as the streets of cities, cafeterias, etc., where there are many different sounds at the same time. This means users are not capable of understanding speech clearly, nor accurately locating and detecting the sound sources.